Affinity Methods Associated with Recombinant Proteins

The most significant development in the past decade for purifying proteins has been the use of molecular biology techniques not only to express the desired protein in large amounts, but also to modify it in a way that makes its purification easy. This is achieved by making the protein longer with extra amino acid residues (polypeptide) that can be recognized by an affinity column. The product is called a fusion protein. There are many systems available, and the process is made easy by commercially available expression systems that automatically add the desired polypeptide. In some cases, the addition may be fewer than ten amino acids. In other cases it is a whole extra protein, which folds into an active shape while linked to the desired protein being expressed. Often the overall expression level is even higher with these 'fusion products' than with the unmodified protein, because the expression of the fusion portion has been optimized in designing the vector.

After purifying the fusion protein, it may be necessary to remove the fusion portion. This can be done with selective proteases, though these usually still leave a few extra amino acids on the end. One system involving an intein, or self-splicing protein, enables release of exactly the desired protein with no gain or loss of amino acid.

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